Composite insulating materials (generally tapes or mats) containing one or more plies of mica paper are well known commercial products. Mica and mica paper are heat resistant up to at least 500.degree. C. and have excellent voltage endurance and corona resistance. Consequently, the advantages of such materials for high-voltage situations are apparent.
The mica paper is utilized in coated or laminate form. These additional layers are employed to improve its physical integrity and other desirable properties. Polymer coating may, for example, be applied to one or both sides of the paper. Certain such polymers may also be utilized as a binder in composite materials made by laminating separate mica paper sheets together and/or to supporting plies of glass cloth, cellulosic paper or other common reinforcing sheet materials.
These composite mica paper materials containing coatings and/or laminates have proven particularly efficatious. They can be provided as tape or wrapper materials readily applied over the structure to be insulated. This greatly facilitates their handling.
A drawback of these composite materials has been the difficulty of their production. Flow of resin after initial application may result in uneven coatings and/or spillage. Also it is often difficult to obtain good layer-to-layer adhesion, particularly while retaining flexibility in the material.